Chemical milling or etching accomplishes reduction of the dimensions of a part by the reaction of the chemical milling solution with the immersed part. Control of this milling operation is critical since it is important that the parts be milled to within a close tolerance. Control of the milling operation, however, has proven problematic. For example, in U.S. Pat. No. 2,933,675, Hoelzle discloses a chemical milling controller which measures the conductivity of the milling solution and automatically calculates the respective milling rate and desired immersion time for parts within the milling solution. This control is problematic because it requires a relatively small milling tank, requires recalibration of the instrumentation for use of different solutions and different alloys, and requires recalibration for the milling of parts of different size and shape. Continuous sensing and analysis of the conductivity must be taken since the conductivity of the solution changes continuously as the part is etched. The rate of change of the conductivity is used to calculate the total depth of milling achieved for the part through analog intergration of the rate curve. Since the rate of change is a function of the ratio of the surface area of the part to the volume of the solution, the system is dependent upon the relative sizes of the tank and the respective parts. Parts of different size and shape will produce different rates of change in the conductivity when etched to equal depths, because the parts will have different volumes of metal removed during the milling.
In U.S. Pat. No. 3,959,046, Bussmann et al. disclose another automated chemical milling controller. There, etching solution is withdrawn from the milling tank and is circulated in an etching supply line. Prior to reentry into the milling tank, the etching solution is analyzed to determine the actual milling rate of the recycled solution by having the recycled solution etch through a wire of known composition and known geometric configuration. Based upon the measured milling rate, the time for immersion of parts within the tank is calculated. This control fails to account for differences in performance between the etching solution in the milling tank and the sample lot in the supply line. The method assumes that the solution is well mixed within the tank and that the etching rate is constant as the parts are etched. For these assumptions to be correct, it is necessary that the parts be small and have a relatively short residence time within the tank so that the volume of metal etched from each part is small. Otherwise, the bath composition will change radically during the milling operation, the milling rate will change, and control will be lost. The controller of U.S. Pat. No. 3,959,046 is inadequate for most milling operations, because the assumptions introduce unacceptably large sampling errors.